A row has broken out between Wikileaks and one of the newspapers it collaborated with to leak US diplomatic cables.
The whistle-blowing group said unredacted versions of the 251,000 diplomatic cables had been leaked on the internet.
Wikileaks blamed the disclosure on the Guardian newspaper and said it had started legal action against the paper.
The newspaper has strongly denied the claims, blaming a "security breach".
In a short statement on Twitter, Wikileaks said: "A Guardian journalist has, in a previously undetected act of gross negligence or malice, and in violation of a signed security agreement with the Guardian's editor-in-chief Alan Rusbridger, disclosed top secret decryption passwords to the entire, unredacted, Wikileaks Cablegate archive.
"We have already spoken to the State Department and commenced pre-litigation action."
Stories based on classified US diplomatic cables - allegedly leaked by US soldier Bradley Manning - have featured in the mainstream media since December 2010 after Wikileaks partnered with the papers including The Guardian and New York Times to release the information.
An unredacted version of the cables is reported to be circulating on the internet and Wikileaks says that a book, published by two Guardian journalists in February, reveals the password to open the file.
The Guardian admits the book contains a password, but says it does not reveal the location of the file and that it was previously told by the Wikileaks' founder, Julian Assange, that the password was temporary and "would expire and be deleted in a matter of hours".
The paper said it "utterly rejects" the suggestion it was to blame for the unredacted version appearing and that it had gone to great lengths to ensure "potentially vulnerable sources" were protected.
The Guardian added: "No concerns were expressed when the book was published and if anyone at WikiLeaks had thought this compromised security they have had seven months to remove the files.
"That they didn't do so clearly shows the problem was not caused by the Guardian's book."
Wikileaks claims the disclosure could have serious consequences and that "revolutions and reforms are in danger of being lost".
"Every day that the corrupt leadership of a country or organisation knows of a pending WikiLeaks disclosure is a day spent planning how to crush revolution and reform," said a statement from the whistle-blowing site.
American officials said on Wednesday that the disclosures could also have serious consequences for informants, human rights activists and others quoted in the cables.
"What we have said all along about the danger of these types of things is reinforced by the fact that there are now documents out there in unredacted form containing the names of individuals whose lives are at risk because they are named," said US Defense Department press director Col David Lapan.
"Once WikiLeaks has these documents in its possession, it loses control and information gets out whether they intend [it] to or not."
The files were originally sent to the Guardian in July 2010 via a secure server which was then wiped, but it says that - unknown to anyone at the paper - the files later ended up on the BitTorrent filesharing site.
It has long been known that WikiLeaks lost control of the cables even before they were published.
One copy of the secret documents was leaked to the New York Times in autumn 2010 and other media organisations have since received copies independently of Wikileaks.
The organisation was also criticised this week for not redacting names as it released another 133,000 US State Department cables.
Australia's Attorney General, Robert McClelland, said the publication of one such cable, naming Australian terror suspects and marked "secret", was "incredibly irresponsible".
Wikileaks denied that any "informants" had been identified in the newly-released files and said the material was "unclassified and previously released by mainstream media".
However, other sources claim the release does contain some classified files where names have not been withheld.